This noise is coming from the engine bay, particularly by the air filter. The noise can be cancelled by moving the valve / flap as shown, but there is something wrong in the engine. Suspect is injector, valve, piston or something else, but without a plug in diagnostic, I'm open to suggestions.
Update: The problem was a worn camshaft and followers. I replaced them with the engine in the car. It took around 5 hours, with the hardest part getting to and removing the tandem diesel and pneumatic pump. The parts I used were https://www.ebay.co.uk/itm/142384737209 plus I bought the special tools, https://www.ebay.co.uk/itm/264656812432.
I hope this helps someone.